1. Field of the Invention
The present invention relates to a voltage type inverter for inverting a DC voltage into an AC voltage and a method of controlling it. More particularly, the present invention relates to a voltage type inverter which gives an output voltage setting command to an inverter circuit having a switching element subjected to pulse width modulation and a method of controlling it.
2. Description of the Related Art
FIGS. 12 to 15 show a prior art. FIG. 12 shows a well-known voltage type inverter device disclosed in JP-A-60-26496. In FIG. 12, reference numeral 1 denotes an A. C. power source; 2 a converter circuit for rectifying the AC power from an AC power source; 3 a smoothing capacitor for smoothing the DC voltage from the converter circuit 2; 4 an inverter circuit for inverting the DC voltage from the smoothing capacitor into an AC voltage having a predetermined frequency; and 5 an induction motor (IM) which is load driven by the inverter circuit 4.
Reference numeral 10 denotes a signal input terminal for receiving a frequency command value F (*) which is a voltage signal corresponding to an output frequency as an output frequency setting signal; 11 an F/V arithmetic unit for creating a voltage command V (*) having a predetermined relationship with the frequency command value as shown in FIG. 13; and 12 a PWM signal generator for receiving the frequency command value F (*) and the voltage command value (*) to generate a predetermined signal for controlling a switching element of the inverter circuit 4.
An explanation will be given of the operation of the voltage inverter device. During a low speed running, in order to compensate for the reduction in an excited current, as shown in FIG. 13, the voltage command value (*) at the frequency command value F(*) of 0 is enhanced by an offset value V0 to satisfy the following equation. EQU V(*)=K.multidot.F(*)+V0
where K is a proportional constant.
In a configuration in which speed control is not carried out as in the prior art, because of small generated torque, as load torque increases, the speed is apt to decrease. In order to obviate this, the offset value V0 is set for a large value to strengthen excitation so that sufficient torque can be generated and hence even when the load torque is large, the speed is not reduced. However, when the excitation is strengthened to set the F/V characteristic in this way, excess excitation results in no load so that a current with no load is extremely increased to generate an excess current.
In order to obviate such a defect, control of slip frequency has been adopted conventionally. FIG. 14 shows a voltage type inverter device using the slip frequency control shown in JP-A-63-144795. In FIG. 14, reference numeral 6 denotes a Hall CT (current transformer) arranged on the DC power line of the inverter circuit 4; 7 Hall CTs for detecting three-phase line currents I (U), I (V) and I (W) to the induction motor; 8 a voltage sensor for sensing the DC input voltage V (DC) to the induction motor; and 9 is a voltage sensor for sensing the line-to-line voltage V (UW) of the induction motor 5.
Reference numeral 13 denotes a slip estimator which receives the DC input voltage V (DC) from the voltage sensor 8, line-to-line input voltage V (UW) from the voltage sensor 9, DC current I (DC) from the Hall CT 6, three phase line-to-line currents I (U), I (V) and I (W) from the Hall Cts 7 to produce a secondary input P2 to the induction motor 5 and a slip frequency estimated value Fs. Reference numeral 14 denotes a voltage corrected amount determiner which receives the secondary input P2 from the slip estimator 13 to produce an output voltage corrected amount .DELTA.V1. Reference numeral 15 denotes an adder which adds the rotary speed command value Fr(*) from the signal input terminal 10 and the slip frequency estimated value Fs from the slip estimator 9 to produce a frequency command value F (*). Reference numeral 16 denotes an adder which adds the output voltage corrected value .DELTA.V1 from the voltage corrected amount determiner 14 and the voltage command value V (*) from the F/V arithmetic unit 11 to create a corrected voltage command V1 (*).
An explanation will be given of the operation of the inverter device of FIG. 14. By the adder 15, the rotary speed command value Fr (*) inputted from the signal input terminal 10 is added to the slip frequency estimated value to produce the frequency command value F (*). The frequency command value F (*) is supplied to the F/V arithmetic unit 11 to create the voltage command value V (*). By the adder 16, the voltage command value V (*) is added to the output voltage corrected value .DELTA.V1 from the voltage corrected amount determiner 14 to create the corrected voltage command value V1(*). The PWM signal generator 12 generates the PWM signal for controlling the switching element of the inverter circuit 4 on the basis of the frequency command value F (*) and the corrected voltage command value V1 (*) to drive the induction motor 5 with a prescribed F/V characteristic.
FIG. 15 shows the detailed configuration of the slip estimator 13. In FIG. 15, reference numeral 20 denotes an IM primary input arithmetic unit for computing a primary input power P1 on the basis of a DC current I (DC) from the Hall CT 6 and the DC input current voltage V (DC) from the voltage sensor 8. Reference numeral 21 denotes a current effective value arithmetic unit for computing an input current effective value I1 on the basis of the three-phase line currents I (U), I (V) and I (W) from the Hall CT 7. Reference numeral 22 denotes a voltage effective value arithmetic unit for computing the input voltage effective value VI on the basis of the line-to-line input voltage V (UW) from the voltage sensor 9. Incidentally, although the current effective value arithmetic unit 21 performs the computation on the basis of the three-phase line currents, it may perform the computation on the basis of the two-phase line currents.
Reference numeral 23 denotes an IM primary copper loss arithmetic unit for computing a primary copper loss W1 of the induction motor 5 from the input current effective value I1. Reference numeral 24 denotes an IM iron loss arithmetic unit for computing an iron loss W0 of the induction motor on the basis of the input current effective value I1 and the input voltage effective V1. Reference numeral 26 denotes an arithmetic unit for subtracting the primary copper loss W1 and iron loss W0 added by the adder 25 from the primary input power P1 to create a secondary input P2. Reference numeral 27 denotes a slip frequency arithmetic unit for computing the slip frequency estimated value Fs on the basis of the secondary input P2.
The prior art for controlling the output voltage from the voltage type inverter by the PWM system is disclosed in JP-A-59-153467 and JP-A-1-99478 in which the output from the inverter device is compared with a sine wave reference voltage and is controlled on the basis of a difference therebetween.
In a control circuit for the prior art voltage type inverter device as described above, since the output voltage from the inverter device is controlled in comparison to the sine wave reference voltage, a difference between the reference sine wave and an actual output voltage does not become zero completely so that a difference in an amplitude and a phase occurs and a normal difference remains. If a sinusoidal wave oscillator for providing a control difference is not inserted in a control system, unbalance in the output voltage or a phase shift due to unbalance in load or voltage or variation in the circuit constant cannot be compensated for.
In a system in which a slip is estimated without directly detecting the speed of an induction motor and using a constant on the primary side of the induction motor like a sensor-less spectrum control, particularly in control in a low speed area, errors in the output voltage occur due to the detecting accuracy of a voltage and a current, short-circuit preventing time of a switching element of the inverter circuit and the "on" voltage of the switching element. This impairs the control characteristic.